Variable resolution radar for tropospheric sounders

ABSTRACT

A radar sounder for plotting and studying the refractive structure of atmospheric strata close to the ground is described. The radar comprises means for linearly modulating and vertically radiating a beam of microwave energy. The beat frequency produced in the receiver by the heterodyning of the outgoing with the returning frequencies, is a function of the elevation of the strata from which the reflections occur. Importantly, the degree of resolution of the radar of this invention can be varied from coarse to fine without altering the duty cycle or the total amount of radiated power. Coarse resolution is preferred for searching for the important principal refractive layers from ground to high altitude, while fine resolutions is necessary for examining the fine structure of the selected layer. Of extreme importance is the ability of the radar sounder of this invention to observe structures close to the ground as this is the range where refractive index variations are the greatest.

United States Patent 1191 Richter 1 1March 13, 1973 I VARIABLERESOLUTION RADAR FOR TROPOSPHERIC SOUNDERS [75] Inventor: Juergen H.Richter, San Diego,

' Calif.

[73] Assignee: The United States of America as represented by theSecretary of the Navy [22] Filed: April 6, 1970 [2]] Appl. No.: 26,008

[52] U.S. Cl. ..343/14, 343/5 SA, 343/5 W [51] int. Cl ..G0ls 9/32 [58]Field of Search ..343/5 SA, 5 W, 14, 17.5

[56] References Cited UNITED STATES PATENTS Primary ExaminerMalcolm F.l-lubler Attorney-R. S. Sciascia, G. J. Rubens, .l. W. McLaren and T. L.Styner [57] ABSTRACT A radar sounder for plotting and studying therefrac- N tive structure of atmospheric strata close to the ground isdescribed. The radar comprises means for linearly modulating andvertically radiating a beam of microwave energy. The beat frequencyproduced in the receiver by the heterodyning of the outgoing with thereturning frequencies, is a function of the elevation of the strata fromwhich the reflections occur. Importantly, the degree of resolution ofthe radar of this invention can be varied from coarse to fine withoutaltering the duty cycle or the total amount of radiated power. Coarseresolution is preferred for searching for the important principalrefractive layers from ground to high altitude, while fine resolutionsis necessary for 2,958,865 ll/l960 Rabier ..343/6 R x examining the fineStructure of the Selected y of 3,273,152 9/1966 Earp 343 103 M extremeimportance is the ability of the radar sounder 3,i9l,174 6/1965 Heisleret al. ..343/17.5 of this invention to observe structures close to the3,l08,273 6 343/14 ground as this is the range where refractive index3,039,088 6/1962 Atlas ..343 5 w variations are greatest 3,344,4199/1967 Lund ..343/5 SA 6 Claims, 7 Drawing Figures b {14 1 A5335? TUNEDnH U R R E PwE |5OW AMPLIFIER SUPPLY l DELAY LINE 5 TIMING *1 43 VARIABLEATTENUATOR AUDIO l OSCILLATOR 1 DELAY LINE 4 L AUDIO OSCILLATOR 2o 24 9Low NOISE BACKWARD FIRE-AMPLIFIER H 3122?, AMP L l FIER K121119125 QINSWEEP DELAYED SWEE? CRT SWEEP slam. REAL-TIME F a sh GENERATOR ANALYZ-ERgaging:

FREQUENCY PATENTEDHAR13 1975 AMPLITUDE AMPEITUDE SHEET 1 OF 3 FlGQlPRIOR ART) t TIME FIG. 2'

(PRIOR ART) TIME FIG. 3

FREQUENCY INVENTOR.

JUERGEN H. RICHTER 5111-1 M (Zixm/ ATTORNEYS PATH-Emma laws 3. 720,949

SHEET 2 BF 3 L l2 A5335? AN QT R 'fiik? 450w AMPUHER OSCILLATOR SUPPLY4O DELAY LINE 5 TIMING (43 VARIABLE ATTENUATOR AUDIO OSCILLATOR DELAYLINE /42' 45 L AUDIO OSCILLATOR 22 26 20 I f {24 LOW NOISE BACKWARD PREVARIABLE PRE-AMPILIFIER, magi AMPLIFIER BANDPASS QM swag K \DELAYEDswEEPK 32 CRT SWEEP SIGNAL REAL'T'ME DISPLAY DISPLAY SPECTRUM 1- l UNITGENERATOR UNIT ANALYZER CIANALYZER I/ ISA INVENTOR.

FIG. 5

' JUERGEN H. RICHTER I PATENTEDMAM 3 i975 SHEET 3 OF 3 TRANSMITTERRECEIVER W ilr w I: l I

FIG. 6

FIG. 7

INVENTOR. E I JUERGEN H. RICHTER BY I ATTORNEYS VARIABLE RESOLUTIONRADAR FOR TROPOSPHERIC SOUNDERS STATEMENT OF GOVERNMENT INTEREST Theinvention described herein may be manufactured and used by or for theGovernment of the United States of America for governmental purposeswithout the payment of any royalties thereon or therefor.

BACKGROUND OF THE INVENTION An atmospheric parameter that affectsmicrowave radio propagation in the troposphere is the refractive index.The structure of the refractive index varies considerably in space andtime. As a result of this variability microwave radio propagation isoften affected in an unpredictable manner. Conventional meteorologicalsounding techniques like radiosonde and microwave refractometer can beperformed at discrete times and places only and consequently do notprovide the continuous information. A sounding technique providing thenecessary continuous information is a ground based microwave radar.PreviOus attempts to sound the troposphere have used existing pulseradar. Because of the minimum range limitations of this type of radarand the difficulty to achieve high range resolution these previoussounding attempts have been of limited value. As this invention hasshown, only the combination of extremely high range resolution and theability to observe at ranges close to the ground revealed a fine scalestructure in the troposphere that had not been expected and not beenobserved before.

OBJECTS OF THE INVENTION The object of this invention is to provide animproved means for finding-and sharply defining refractive layers in thetrophosphere and for displaying finegrain details of the entiretroposphere above the point of observation.

A more specific object of this invention is to provide a radarsurveillance system which is variable in resolution.

SUMMARY OF THE INVENTION The method disclosed herein for measuring anddisplaying the. fine scale refractive index therefore comprises the stepof repetitively linearly modulating, over a given band, the frequency ofa microwave generator. The FM-CW energy is vertically radiated, andreflected energy is picked up and mixed with a suitable amount of thelocally generated transmitted energy to produce a beat frequency thefrequency of which is a linear function of the distance to thereflecting point from which the echo was obtained. By varying the totalfrequency excursion, the resolution of the display can be varied fromcoarse to very fine. Differences in refractive layers as close as 1meter apart can clearly be displayed according to this invention.

Other objects and features of this invention will become apparent tothose skilled in the art by referring to the specific embodimentdescribed in the following specification and illustrated in theaccompanying drawings in which: I

FIGS. 1 and 2 show the curves of important waves generated in prior artfrequency modulation radar devices;

FIG. 3 is a spectrum of the beat frequency which can be found whenanalyzing the signal of FIG. 2;

FIG. 4 is a block diagram of one radar transmitterreceiver according tothis invention;

FIG. 5 is a section of the antenna system employed with thetransmitter-receiver of FIG. 4;

FIG. 6 is an alternative antenna system useable with the transmitterreceiver of FIG. 4; and

FIG. 7 is an example of an A-[ (Amplitude-Intensity) presentation on thescope of the receiver of FIG. 4.

DESCRIPTION OF THE PREFERRED EMBODIMENTS To discuss best the features ofthis invention, the principles of operation of frequency-modulated radarshould briefly be recalled. Referring to FIG. 1, a microwave signal islinearly changed in frequency, see Line A, and is transmitted during theperiod T Any reflected signal is also frequency modulated, the frequencyramp B being delayed Ar, according to the distance of the reflectingobject.

These outgoing and incoming signals are mixed instantaneously whichyield a difference or beat frequency f proportional to time delay At, ordistance. With a suitable spectrum analyzer, a spectrum of the typeshown in FIG. 3 can be obtained. In case of multiple reflecting points,reflected waves will arrive at different time intervals and will causedifferent beat frequencies. It follows that the resolution of the systemdepends on the ability of the system to distinguish between adjacentfrequency spectra of the type shown in FIG. 3. The amplitude of eachspectrum is a measure of the reflection coefficient of each target.

As stated, range resolution means the ability to separate adjacentspectra. The minimum resolvable distance, it, is given by h=c/kF, whereF is the frequency excursion or range of frequencies, from f, to f,, cis the velocity of propagation, and k is a constant whose value dependson the criteria used to define the separation of spectra. Resolution isdependent on the frequency excursion of the transmitted signal, andaccording to an important feature of this invention is varied withoutchanging the transmitted power of the system.

The transmitter shown in FIG. 4 comprises an oscillator 10 coupled tothe pre-arnplifier 12, to the power amplifier l4 and hence to thedirectional antenna with reflector 16. The preferred technique ofmodulating the frequency of the oscillator comprises a YIG sphere in thefrequency determining circuit of the oscillator. This sphere composed ofyttrium, iron and garnet, is particularly effective in obtainingstraight line frequency change from a direct current voltage ramp. Thenatural frequency of the YIG sphere varies linearly with the magneticfield in which it is enclosed. The sweeping current supply 8 supplies alinearly variable magnetic field to the YIG sphere. Timing of the entireradar system including the initiation of excursions of the directcurrent supply is by the timing device 6. According to this inventionthe range of frequency F is extended or contracted at either or bothends merely by adjusting in supply 8 the length of the direct currentvoltage ramp. applied to the frequency determining circuit of oscillator10.

All received signals at the antenna 18 are amplified at preamplifier 20,and converted to the beat frequencies at the backward diode mixer 22.The beat frequencies are amplified at the preamplifier 24. The variableband pass filter 26 passes a selected band of the signal to the realtime spectrum analyzer 28. Then a running average of the signals isachieved through digital averaging techniques in the signal analyzer 30.The signals are displayed on cathode ray tube display units 32 and 34.

Analysis of the spectrum of the beat frequency must be performed in realtime which can be done with any real time spectrum analyzer, one ofwhich is made generally available from the Federal ScientificCorporation, in New York, N. Y. The raw-signal is sampled at a rate atleast twice the highest frequency of interest and the repetitive signalsare time compressed. Time compression can be effected by a glass delayline in which the output is fed back to the input of the delay line sothat any signal that is fed into the delay line can be recirculated andstored indefinitely. It may take 50 milliseconds, for example, to fillthe register with realtime information, this time being determined bythe modulation time T FIGS. 1 and 3. Yet all the information in theregister can be read out and processed in, say, 0.10 millisecond. Thatis, time compression can be quite high. During T the memory is updatedand the analysis starts at the end of T when the memory is put intohold. The input signal was time compressed by a factor of 500 in oneembodiment, when the scanning spectrum analysis was completed in 50milliseconds. After analysis time a new modulation cycle starts, and theentire cycle of frequency sweep from f, to f,, followed by storage andspectrum analysis can conveniently be repeated at a rate of per second.By multiplying the output of a spectrum analyzer with a sine squaredweighting function the side lobes of the sin x/x spectrum, shown in FIG.3 is suppressed, although the spectrum is somewhat broadened and theresolution bandwidth is about 30 Hz.

According to an important feature of this invention, portions of thetransmitted microwave energy is fed through two paths into the receivingchannel. The first path includes a delay line 40 feeding transmittedenergy directly into the backward diode mixer 22. Delay line 40 isadjusted to produce a delay equal to the combined delays created by thepower amplifiers and the cables to and from the antennas. That is, thedelay line 40 has exactly the length or delay to produce a zero beatfrequency with the signal which would be created by direct coupling thetransmitting antenna 16 to the receiving antenna 18. Thesecond pathincludes delay line 42 which couples part of the transmitted energy intothe low noise preamplifier 20. The length of delay line 42 is carefullyadjusted to equal the round trip transit time of a radar signal to atarget at any arbitrary height, such as 167 meters employed in onesystem. The reference signal created in this matter is a check of allactive components of the system and giving a convided with tapscarefully spaced apart electrical distances correspond to free-spaceranges for radio waves of fixed amounts, such as l, 2, and 10 meters.

The output of the spectrum analyzer 28 is fed into the signal analyzer30 which averages several consecutive sweeps. The number of sweepsaveraged can be selected and determines the time constant introduced tothe signal.

The output of the signal analyzer is applied preferably to two cathoderay tube display units 32 and 34. The horizontal sweep for the displayunits is provided by the sweep generator 36. A delayed sweep is appliedto cathode ray tube 32 which delays the sweep with respect to the mainsweep applied to CRT 34 which allows one to view simultaneously theentire analysis band and any enlarged portion thereof. One of the CRTsmay be a storage scope on which the time history of the reference can bedisplayed as real time up to 1 hour.

The two audio oscillators 44 and 46 provide variable height markings onthe display screen for readily identifying altitudes in meters or feet.i

For maximum isolation of the transmitted and received microwave energy,separate antennas should be used. In FIG. 5, for example, the antennasl6 and 18 consist of two parabolic reflectors, called "dishes, about 3meters in diameter and 4 meters between centers. In order to achievegood isolation the antennas are located in separate pits in the ground.Metal shields around the outside of the pits and microwave absorbent atthe inside increase further the isolation. The db isolation, far inexcess of the isolation necessary to avoid saturation of the receiverpreamplifiers, was easy to attain in practice by the pit constructionand the absorbing sources. The antennas must be steerable within alimited degree, such as 2.5 from vertical, in order to obviate parallaxand to optimize a common volume of intersection at a given altitude.Desirably, at least one of the reflectors should be remotely tiltablewith respect to the other so that the zone of intersection of the twobeams can be raised or lowered. In FIG. 5 is suggested one techniquewhere the entire antenna and reflector assembly 16 is mounted on threescrew jacks 16a evenly spaced about the edge of the dish reflector. Areversible motor, not shown, can be employed to drive one or more of thescrews to remotely tilt the dish, if desired. The other reflector 18 canbe stationary and accurately aimed vertically, or may be supplied with atilting mechanism. The range of motion of the reflectors from theverticle can be quite small to bring the beams into coincidence at lowas well as high altitude. As stated, a range of motion of 12.5 is ample.

Alternatively, a single antenna and directional parabolic dish withsingle or cross polarized feeds may be used as shown in FIG. 6 with thecirculator 54 for routing the output of the transmitter to the antennaand for routing the signals from the antenna to the receiver. Thisarrangement commonly provides about 25 40 decibels separation betweenthe transmitted and the received signals.

The following table sets forth the parameters and performancecharacteristics of one specific radar system shown in FIG. 4 used formeteorological surveying according to this invention.

Table 1. Performance Characteristics of the Radar Sounder PARAMETERVALUE REMARKS Power 150 Watts Center frequency 2.8 3.1 GHZ Frequencyexcursion variable Linear modulation Range resolution variable Maximumresolution 1 m for 219 MHZ frequency excursion Sweep duration 50 msec 10sweeps per second Receiver noise figure 5 dB Minimum detectable signall50 dBm Antenna beam width 2.3 degrees Isolation between antennas 105 dBMinimum detectable cross section at 1 km -31 X 10"sq. cm.

Minimum detectable reflectivity at 1 km -42 X l"cm1 for l in rangeresolution It is apparent, of course, that many ranges of frequenciesmay be employed within the capabilities of the components used. Therestriction to 2.9 gigahertz was determined by the standing wave ratio,SWR, of the antenna feed and the balanced mixer.

As stated, the variable range resolution is an important feature of thisinvention. In the case of volume scattering the signal amplitude changescontinuously with the resolution thereby permitting significantconclusions to be drawn. The maximum range resolution of about 1 meterwas determined experimentally. The spectra of two targets 93 centimetersapart were separable using a frequency excursion F of 219 megahertz in50 milliseconds. Insects with a cross-sectional area of 10-3 squarecentimeters can cause dot echoes 24 db above noise at a height of lkilometer.

FIG. 7 illustrates the combined amplitude intensity display recorded onan expanded time basis. Each vertical trace on the recording is arunning average of the Signal on the previous 32 frequency sweeps withan exponential eighting function and a time constant of 3.2 secondsevery 16th trace is displayed so that the record shows 37.5 sweeps perminute. In this way the signal amplitudes can be read quantitativelyusing the powerdeflection chart of FIG. 7 without overlap of adjacenttraces. By using the signals themselves to provide simultaneousintensity modulation the record is shaded so that the brightest echoesare also the strongest. The fine structure of the refractive layers andtheir intensity can be obtained with the amplitude-intensity display,which is especially valuable for meteorological forecasting.

Close examination of FIG. 7 indicates that there are two distinctrefractive layers C and D. The bottom layer thickens progressively withtime from about meters at the start, not shown, to a maximum of about 35meters 4 minutes later at about 1,213 hours, Pacific Standard Time. Theapparent broadening of the stratum just after 1,212 PST appears to bedue to a jump in the layer height. Careful scrutiny shows that a singlerefractive layer occasionally had two welldefined intensity peaksseparated vertically 5 to meters.

It is clear that the FM radar above disclosed has an excellentcapability for detecting the fine scale refrac tivity inhomogeneitieswhich occur at atmospheric interfaces showing sharp refractivitygradients or changes in gradient. Not only can the entire troposphere besimultaneously displayed for a good perspective but the high resolutionof the radar permits examination of minute details of the strata.

Obviously many modifications and variations of the present invention arepossible in the light of the above teachings. It is therefore to beunderstood that within the scope of the appended claims the inventionmay be practiced otherwise than as specifically described.

What is claimed is:

1. in measuring and displaying the altitude and details of strata ofdistinct refractive indices of the troposphere, the method comprisingthe steps of:

generating continuous microwave energy;

repetitively linearly modulating the frequency of the microwave energy;

vertically radiating from a directional antenna a relatively narrow beamof said microwave energy;

focusing and vertically fixedly directing directional receiving antennameans to substantially the same space illuminated by the radiated beamto receive any microwave energy reflected vertically downward fromoverhead atmospheric strata;

combining outgoing and reflected energy in a low flicker noise mixer togenerate beat frequencies which instantaneously are functions of thealtitude of said reflecting strata;

displaying said beat frequencies in terms of altitude;

and

varying the range of frequencies of said microwave energy for varyingthe resolution of the display.

2. in combination in a system for displaying a cross section of therefractive strata of the troposphere, said system comprising:

a microwave generator with a frequency determining circuit;

a frequency determining means having a resonant frequency which, over apredetermined range, is a linear function of a variable direct current;

a sweep current supply for generating a succession of straight-linevoltage ramps;

means for applying said voltage ramps to said frequency determiningmeans for linearly varying the microwave frequency of said generator;

stationary antenna means for forming a relatively narrow beam, saidantenna means being disposed to aim vertically said beam,;

plural cascaded amplifier stages coupling the output of said generatorto said antenna means;

a low flicker noise mixer of microwave frequencies;

plural cascaded stages coupling the receiver terminal of said antennameans to one input of said mixer, the other input of said mixer beingcoupled to the output of said generator for combining outgoing andincoming microwave energy for generating beat frequencies; and

display means for showing said beat frequencies, said display showingsaid beat frequencies in terms of the vertical height of atmosphericstrata from which said energy is reflected.

3. The combination defined in claim 2 comprising further: I

a delay line coupled in the circuit between the output of said generatorand said mixer, the amount of delay being adjusted to approximatelyequal the total delays of all amplifiers and signal circuitry betweensaid generator, said antenna means and said mixer. 4. The combinationdefined in claim 2 further comprising:

means for selectively varying the length of said ramps for varying atwill the excursions of the frequency of said microwave generator toalter the resolution of said system. 5. The combination defined in claim2 further comprising:

an adjustable delay line coupled between the transmitter terminal andthe receiver terminal of said antenna means, the amount of said delaybeing adjustable to amounts corresponding to predeter-

1. In measuring and displaying the altitude and details of strata ofdistinct refractive indices of the troposphere, the method comprisingthe steps of: generating continuous microwave energy; repetitivelylinearly modulating the frequency of the microwave energy; verticallyradiating from a directional antenna a relatively narrow beam of saidmicrowave energy; focusing and vertically fixedly directing directionalreceiving antenna means to substantially the same space illuminated bythe radiated beam to receive any microwave energy reflected verticallydownward from overhead atmospheric strata; combining outgoing andreflected energy in a low flicker noise mixer to generate beatfrequencies which instantaneously are functions of the altitude of saidreflecting strata; displaying said beat frequencies in terms ofaltitude; and varying the range of frequencies of said microwave energyfor varying the resolution of the display.
 1. In measuring anddisplaying the altitude and details of strata of distinct refractiveindices of the troposphere, the method comprising the steps of:generating continuous microwave energy; repetitively linearly modulatingthe frequency of the microwave energy; vertically radiating from adirectional antenna a relatively narrow beam of said microwave energy;focusing and vertically fixedly directing directional receiving antennameans to substantially the same space illuminated by the radiated beamto receive any microwave energy reflected vertically downward fromoverhead atmospheric strata; combining outgoing and reflected energy ina low flicker noise mixer to generate beat frequencies whichinstantaneously are functions of the altitude of said reflecting strata;displaying said beat frequencies in terms of altitude; and varying therange of frequencies of said microwave energy for varying the resolutionof the display.
 2. In combination in a system for displaying a crosssection of the refractive strata of the troposphere, said systemcomprising: a microwave generator with a frequency determining circuit;a frequency determining means having a resonant frequEncy which, over apredetermined range, is a linear function of a variable direct current;a sweep current supply for generating a succession of straight-linevoltage ramps; means for applying said voltage ramps to said frequencydetermining means for linearly varying the microwave frequency of saidgenerator; stationary antenna means for forming a relatively narrowbeam, said antenna means being disposed to aim vertically said beam,;plural cascaded amplifier stages coupling the output of said generatorto said antenna means; a low flicker noise mixer of microwavefrequencies; plural cascaded stages coupling the receiver terminal ofsaid antenna means to one input of said mixer, the other input of saidmixer being coupled to the output of said generator for combiningoutgoing and incoming microwave energy for generating beat frequencies;and display means for showing said beat frequencies, said displayshowing said beat frequencies in terms of the vertical height ofatmospheric strata from which said energy is reflected.
 3. Thecombination defined in claim 2 comprising further: a delay line coupledin the circuit between the output of said generator and said mixer, theamount of delay being adjusted to approximately equal the total delaysof all amplifiers and signal circuitry between said generator, saidantenna means and said mixer.
 4. The combination defined in claim 2further comprising: means for selectively varying the length of saidramps for varying at will the excursions of the frequency of saidmicrowave generator to alter the resolution of said system.
 5. Thecombination defined in claim 2 further comprising: an adjustable delayline coupled between the transmitter terminal and the receiver terminalof said antenna means, the amount of said delay being adjustable toamounts corresponding to predetermined altitude-ranges.